[22.07] Cloud structure on Uranus as constrained by near IR 1.1-1.8 micron spectra.

L. A. Sromovsky, P. M. Fry (University of Wisconsin - Madison)

Three uranian cloud layers were identified by West et al.
(Uranus, Univ. Arizona Press, 1991): an optically thin
stratospheric haze, an optically thicker methane haze (0.4
< \tau < 1) primarily in the 1.2-1.3 bar region, and a
cloud of unknown composition near 3 bars. Using improved
methane band models of Irwin et al. (BAAS, this issue)
we were able to test this paradigm using near-IR spectra
covering 1.1-1.8 \mum, a range well suited for
distinguishing the main cloud levels. We assumed a 2-cloud
model in which the lower cloud is opaque and the upper cloud
consists of broken opaque elements. The pressure and
fractional coverage of the upper cloud and the pressure and
albedo of the lower cloud were adjusted to fit the 1975
geometric albedo spectrum of Fink and Larsen (ApJ \bf
233, 1021-40, 1979), with the following results (first two
rows):

Fit Range (\mum)

Upper P (bars)

Upper Fraction(%)

Lower P (bars)

Lower Albedo (%)

\chi2

1.175-1.34

2.2±0.15

2.8±0.4

6.6+1.2-0.7

6.5±0.8

206

1.450-1.70

1.8±0.10

2.2±0.3

5.2+0.8-0.4

3.3±0.3

223

1.175-1.34

1.25 (fixed)

0.15±0.08

3.1 (fixed)

6.6±0.8

296

1.450-1.70

1.25
(fixed)

0.66±0.05

3.1 (fixed)

4.4±0.1

281

Fixing clouds at paradigm pressures of 1.25 bars and 3.1
bars yields a significant reduction in fit quality and a
very small upper cloud contribution (last two rows). The
paradigm-violating best-fit results are consistent with an
analysis of seven-band Keck AO imaging observations
(Sromovsky and Fry, in preparation), which concludes that
the 1.2-bar cloud is at best a minor contributor to Uranus'
reflectivity and that latitudinal variations in brightness
are mainly controlled by deeper clouds. How prior results
can be explained in the context of these new results remains
to be determined.

This research was supported by a grant from NASA's Planetary
Astronomy Program.